The overall objective of this research project is to acquire a better understanding of the mechanisms of arrhythmias by studying spontaneous supraventricular tachycardia. Spontaneous initiation and termination of supraventricular tachycardia will be recorded by ambulatory ECG using a new bipolar recording electrode that can be swallowed and retained in the esophagus by thin lead wires that are taped to the chin. Once taped in place, this electrode remains stable for many days and does not interfere with talking, eating or sleeping. Unlike the low-voltage P wave recorded on surface leads, the bipolar esophageal lead records a high amplitude, rapid atrial electrogram that can be used on time atrial events. This signal overcomes the technical problem of recording P waves on ambulatory ECGs. Timing atrial depolarization from the esophageal lead will simplify computer algorithms for diagnosis of arrhythmias. A new computer system of analysis of 2 channel ambulatory ECGs using the esophageal lead and one surface lead will increase the accuracy of computer interpretation while decreasing the cost. This new recording and processing system will be used to screen patients before additional diagnostic testing with intracardiac recordings, and to document the mechanism of action of antiarrhythmic drugs.